The invention relates to intraocular lenses and more specifically to a new process for improving the ductility and fatigue resistance of the PMMA (polymethyl methacrylate) that is machined to form one-piece lenses.
Intraocular lenses are small plastic lenses that are inserted into the interior of the eye after the cataractous natural lens has been removed. The concept of the modern intraocular lens was introduced shortly after the Second World War by the English cataract surgeon and inventor Harold Ridley. His observations of eye trauma in British pilots caused by plastic canopy shrapnel revealed that the acrylic plastic material caused few complications such as infection or foreign body rejection. He concluded that acrylic might make a good material for an artifical lens, and it has in fact proved to be very biocompatible in the eye for long periods.
Modern intraocular lenses have been made by several methods, most notably lathe turning and injection or compression molding. Initially, the optic was made as a separate piece from the haptics, which are flexible supporting structures that act as a centering spring to located the lens within the eye. The haptic filaments in this type of lens (three-piece construction) are made from a monofilament of either PMMA or Polypropylene suture material, which is heat formed to the desired shape. The haptics are attached to the lens by inserting them in a microscopic hole in the lens edge and heat staking them in place. Lenses made this way have proven excellent in service with good flexation characteristics.
More recently, with the advent of very accurate computer controlled machinery, it has become practical to machine the entire lens from one piece of plastic material. This is usually done with a combination of lathe turning operations combined with very precise milling of the profiles necessary to produce the lens shape. Among the advantages of one-piece construction over the earlier three-piece methods (separate optic/haptics) are: fewer pieces, therefore lower labor and fabrication costs, also less concern arout the strength of connections between components, and the one-piece design has fewer places for possible entrapment of pyrogenic materials on the lens.
The material most commonly used for one-piece IOL (intraocular lens) construction is cast PMMA sheet. It is commonly known as "acrylic" plastic, and is characterized by good optical clarity and rigid mechanical properties. Some of its advantages over the other optical materials are its proven compatibility as an ocular implant material, good optical properties including UV light absorption, and mechanical rigidity which makes possible the machining of highly polished and accurate optical surfaces.
The one-piece lens manufacturing method also has some disadvantages. The size of an IOL is quite small, with optic diameters typically 5 to 7 millimeters in diameter, and overall diameter of the haptic structure about 12 to 15 millimeters. The width and thickness of the haptic filaments is about 0.17 millimeters (0.0065 inches ), and consequently these structures are very delicate and fragile. Further, since the PMMA material from which they are machined is a cast polymer, it has isotropic mechanical properties (generally the same strength in any direction). Just as a cast piece of metal is weaker than a metal extrusion or forging, the one-piece lens haptics are weaker than haptics used on three-piece lenses, which are much tougher since they are made from a drawn filament material which is very tough and flexible.
The haptics are most susceptible to weakening and breakage during the implantation proceedure when the surgeon must deflect the loops considerably to insert the lens into the eye. Even after a lens is implanted, the haptics must continue to bear a certain amount of stress exerted within the eye from muscles and outside forces (i.e. rubbing the eye). It is therefore very desirable to have a material with flexibility, toughness, and fracture resistance to make a more durable one-piece lens.
Several inventions relating to contact lenses and intraocular lenses have been granted U.S. patents. The Wichterle U.S. Pat. No. 3,361,858 is directed to a method for reshaping a xerogel by mechanical removal and swelling to form a hydrogel contact lens. The Shepherd U.S. Pat. No. 4,208,364 is directed to a process for producing contact lenses. The Haardt et al U.S. Pat. No. 4,407,776 is directed to a mold and proceedure for producing truncated contact lenses. The Hwang U.S. Pat. No. 4,786,444 is directed to a method and apparatus for making optical devices such as intraocular lenses. The Akhavi U.S. Pat. No. 4,847,020 is directed to a process of molding an optic for an intraocular lens.